1. Field of the Invention
The present invention relates to an acceleration sensor and an acceleration detecting device. More particularly, the present invention relates to an acceleration sensor of the piezoelectric bimorph type and an acceleration detecting device using the acceleration sensor.
2. Description of the Related Art
Various acceleration sensors using bimorph type piezoelectric element are known. For example, in Japanese Unexamined Patent Publication No. 6-324073, an acceleration sensor 51, as shown in FIG. 5, is disclosed.
In the acceleration sensor 51, piezoelectric plates 52, 53 are used. The piezoelectric plates 52, 53 are of a rectangular shape, and the central portions 52a, 53a viewed from the longitudinal direction are polarized in opposite directions from each other in the thickness direction, as shown by the arrows.
Further, external areas 52b, 52c, shown by broken lines, are polarized, as shown by the arrows, in the thickness direction so as to be opposite to that of the central area 52a. Also in the piezoelectric plate 53, external areas 53b, 53c arranged on both sides of the central area 53a, and shown by broken lines are polarized, as shown by the arrows, in the thickness direction so as to be opposite to the central area 53a. Accordingly, the external area 52b and external area 53b, and the external area 52c and external area 53c facing each other are polarized in the thickness direction so as to be opposite to each other, respectively.
On each of the external main surfaces of the piezoelectric plates 52, 53, signal output electrodes 54, 55 are formed respectively. Further, at the portion where the piezoelectric plate 52 and piezoelectric plate 53 facing each other are joined and connected an intermediate electrode 56 is formed. However, the intermediate electrode 56 is formed so as not to extend to both ends of the piezoelectric plates 52, 53.
On the other hand, the signal output electrode 54 is led out to one end of the piezoelectric plate 52, and the signal output electrode 55 is led out to the end portion opposite to the side to which the signal output electrode 54 is led.
On the outsides of the piezoelectric plates 52, 53, supporting members 57, 58 are joined and connected respectively. The supporting members 57, 58 support the piezoelectric plates 52, 53 in the vicinity of both ends of the plates.
Further, on one end surface of the piezoelectric plates 52, 53 and the supporting members 57, 58, an external electrode 59 is formed, and the external electrode 59 is electrically connected to the signal output electrode 55. In the same way, also on the end surface opposite to the side with the external electrode 59, an external electrode (not illustrated) is formed, and this external electrode is electrically connected to the signal output electrode 54.
In the acceleration sensor 51, when acceleration acts in the direction of an arrow A, the piezoelectric plates 52, 53 are bent and the electric charge produced by the bending is output through the electrodes 54, 55, and accordingly the acceleration is able to be detected. As the acceleration sensor 51 is constructed in such a way that the piezoelectric plates 52, 53 are supported in the vicinity of both ends, the amount of electric charge to be generated at the time when acceleration acts is increased and, because of this, even if the acceleration sensor 51 is made of small size, it is said that the detection sensitivity is not likely to be lowered.
Although it is possible to make the acceleration sensor 51 small-sized and to improve the detection sensitivity, the capacitance between the electrodes is small and accordingly there is a problem that acceleration at low frequencies is difficult to be measured. This is explained in detail below.
When acceleration acts in the direction of the arrow A, the generated voltage and the capacitance between the signal output electrode 54 and the intermediate electrode 56, between the intermediate electrode 56 and signal output electrode 55, and between a pair of external electrodes are represented by V1, V2, Vp, C1, C2, and Cp respectively as shown in Table 1.
Here, suppose the thickness, length, and width of the piezoelectric plates 52, 53 are the same, the relation of V1=V2 and C1=C2 results. Therefore, when V1 and V2 are respectively represented by V0, and C1 and C2 are respectively represented by C0, since the piezoelectric plates 52, 53 are connected in series, the generated voltage Vp at the time when acceleration acts in the direction of the arrow A in the acceleration sensor 51 becomes Vp=2 V0, and the capacitance Cp becomes Cp=C0/2.
When acceleration is detected by using the above acceleration sensor 51, because the acceleration sensor 51 has a relatively high impedance, it is common to use a voltage amplifier or a charge amplifier. FIG. 6 is a circuit diagram showing an acceleration detection circuit having such a voltage amplifier connected.
In FIG. 6, a leak resistor R is connected in parallel to the acceleration sensor 51. Further, the output side of the acceleration sensor 51 is connected to one input terminal of a voltage follower 60. Further, the output terminal and the other input terminal of the voltage follower 60 are connected.
In the above acceleration detecting device, the following relation is established: Output voltage VOUT=Input voltage to the amplifier Vi=Generated voltage Vp in the acceleration sensor. The output of the voltage follower 60 converts the output to a sufficiently low impedance.
However, in an operational amplifier and FET constituting the above voltage follower 60, for example, because there is bias current iB flowing out of the input terminal, the above-mentioned leak resistor R is required. That is, unless the leak resistor R is present, the capacitance of the acceleration sensor 51 continues to be charged and the voltage becomes saturated. Accordingly, the leak resistor R is required.
But the leak resistor R causes the electric charge generated at the piezoelectric plates 52, 53 to leak. That is, when the acceleration is slowly changed, or when the acceleration is not changed, the electric charge completely leaks before any voltage Vp is generated. Therefore, no predetermined detection voltage can be obtained. This is expressed by a frequency characteristic as shown in FIG. 7.
FIG. 7 shows the relation between the frequency of acting acceleration at the time when the above-mentioned acceleration detecting circuit is used and the voltage Vi to be input to the voltage follower 60.
In FIG. 7, fc represents a cutoff frequency. Here, the cutoff frequency fc is given by
fc=1/(2xcfx80RCp)
Therefore, in order to measure acceleration at lower frequencies than the above cutoff frequency fc, resistance R and/or capacitance Cp is required to increase. But if resistance R is increased, the offset voltage of the voltage follower 60 increases and in order to reduce the offset voltage an operational amplifier having small bias current is required to be used as a voltage follower, which results in high cost.
Further, even if an operational amplifier of low bias voltage is available, when, for example, a high leak resistance R exceeding 10 Mxcexa9 is connected, advanced measures for humidity resistance are required including for the printed-circuit board to which the leak resistor R is connected. As a result, there are various restrictions even if the resistance of the leak resistor R were to be increased.
On the other hand, the capacitance Cp is determined by the configuration of the piezoelectric plates 52, 53 and the dielectric constant xcex5 of the material constituting the piezoelectric plates 52, 53. That is, the capacitance Cp is given by
Cp=xcex5Wxc2x7L/T
where W, L, and T represent the width, length, and thickness of the piezoelectric plates 52, 53, respectively.
But when the thickness of T is made thinner, the mechanical strength is lowered, and accordingly there is a limit to which the thickness of T may be thinned. Therefore, up to now, in order to increase the capacitance Cp, the width W and/or length L was required to be increased. However, such a method brings about acceleration sensors 51 of larger external dimensions and higher cost.
Also, when a charge amplifier is used, as shown in FIG. 8, leak resistance R and capacitance C are required to be connected in parallel with the operational amplifier 61. And when acceleration at low frequencies was measured, it was required to increase the above leak resistance R and capacitance C. But, since the output of the amplifier VOUT is given by VOUT=Qp/C, the capacitance C was not able to be increased over a certain level in order to obtain a larger output voltage. Here Qp represents electric charge.
Another known acceleration sensor is an acceleration sensor 71 which, as shown in FIG. 9, includes two piezoelectric elements connected in parallel. More specifically, in the acceleration sensor 71, two piezoelectric plates 72, 73, which have been polarized in the thickness direction, are joined with adhesive. On the upper surface of the piezoelectric plate 72 a signal output electrode 74 is formed, and on the lower surface of the piezoelectric plate 73, a signal output electrode 75 is formed. On the joining surface of the piezoelectric plates 72, 73 an intermediate electrode 76 is formed.
Here, conductive patterns 78, 79 are formed on the substrate 77 on which the acceleration sensor 71 is to be mounted. To the conductive pattern 78, the signal output electrode 75 is connected. In like manner, the signal output electrode 74 is connected to the conductive pattern 78 via a lead wire 80. The intermediate electrode 76 is led out from between the piezoelectric plates 72, 73, and connected to the conductive pattern 79 via a lead wire 81.
The signal output electrode 75 is joined and connected to the conductive pattern 78 in the vicinity of one end of the substrate. In this manner the acceleration sensor 71 is supported like a cantilever.
In the above acceleration sensor 71, as the piezoelectric elements using piezoelectric plates 72, 73 are connected in parallel, the capacitance is able to be increased. However, because the acceleration sensor 71 is supported like a cantilever, there is a problem, as explained above, in that the electrical connection needed to obtain a signal from the signal output electrodes 74, 75 and intermediate electrode 76 becomes very complicated and costly.
Further, in the above acceleration sensor 71, by adding more capacitors, the capacitance is able to be increased. Such a circuit is shown in FIG. 10.
As shown in FIG. 10, a capacitor Ca is connected in parallel with the acceleration sensor 71. At a latter stage of the acceleration sensor 71 and capacitance Ca, a leak resistor R and voltage follower 60 are connected in the same way as shown in FIG. 6. Here, when the capacitance of the capacitor Ca is set at three times as large as that of the capacitance Cp of the acceleration sensor 71, the cutoff frequency becomes fc=1/(2xcfx80Rxc2x74Cp) to reduce the cutoff frequency fc to one fourth. However, in this case, the generated voltage Vi becomes the following:
Vi=Qp/(4Cp)=V0xc2x7Cp/(4xc2x7Cp)=V0/4.
That is, the generated voltage Vi is reduced to one fourth.
As described above, the conventional acceleration sensor 51 of the connected-in-series type is comprised of the joined piezoelectric plates 52, 53. And, although acceleration sensors having improved sensitivity and of small size were able to be devised because the piezoelectric plates 52, 53 are supported in the vicinity of both ends of the plates, there was a problem that the acceleration sensors had difficulty detecting acceleration at low frequencies and, when it is attempted to detect acceleration at low frequencies, acceleration sensors of small size cannot be economically devised.
Further, in the conventional acceleration sensor 71 of the connected-in-parallel type, as the sensor is supported like a cantilever, there were problems of low resistance to mechanical shocks, complicated electrical connection for getting signals, and high cost. In addition, when it was attempted to increase the capacitance by connecting an external capacitor Ca and to detect acceleration at low frequencies, there was a problem that the generated voltage Vi was decreased.
It is an object of the present invention to provide an acceleration sensor and acceleration detecting device which solve the above-mentioned drawbacks of the conventional art, and enable the acceleration sensor and the acceleration detecting device to be made small-sized, to have increased capacitance, to be able to detect acceleration at low frequencies with high precision, and to have high charge sensitivity.
According to one aspect of the present invention, an acceleration sensor comprises a strip-like piezoelectric member having first and second end portions, first and second signal output electrodes formed on a pair of main surfaces facing each other of the piezoelectric member, an intermediate electrode formed so as to face the first and second signal output electrodes at the intermediate position between the pair of main surfaces of the piezoelectric member, and first and second supporting members attached to the first and second main surfaces of the piezoelectric member respectively so as to sandwich and support the piezoelectric member at both ends, wherein the piezoelectric member is polarized at the central area in the thickness direction thereof, and the first and second signal output electrodes are led to the first end portion, and the intermediate electrode is led to the second end portion.
In a second aspect of the present invention, polarization is provided in the thickness direction to the central area of the piezoelectric member, and both external areas between which the central area is located in the length direction of the piezoelectric member are polarized in the thickness direction in the opposite direction from the polarization direction of the central area.
In a third aspect of the present invention, the piezoelectric member is made in a strip-like shape and includes a pair of piezoelectric ceramic plates. A signal output electrode and an intermediate electrode are formed on the main surfaces of each of the plates respectively, and electrodes on the plates are joined and connected so as to face each other and to form the intermediate electrode.
According to a fourth aspect of the present invention, the piezoelectric member and the first and second supporting members are joined and connected to form a main body of the acceleration sensor, and first and second external electrodes are formed on the first and second end surfaces on the sides of the first and second end portions of the piezoelectric member of the main body of sensor.
According to a fifth aspect of the present invention, an acceleration detecting device comprises the acceleration sensor as described above, a leak resistor electrically connected in parallel to the acceleration sensor, and an amplifier to amplify the voltage across the leak resistor.
According to a sixth aspect of the present invention, an acceleration detecting device comprises the acceleration sensor as described above, and a charge amplifier connected to the output of the acceleration sensor.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.